1. Field of the Invention
The invention relates to a printing unit with reversible image setting and digital changeover by means of thermal transfer, including an image setting unit, an erasing or cleaning component, and at least one inking unit.
2. Description of the Related Art
As is known, the printing unit, in particular of a sheet-fed offset printing machine, generally comprises the forme cylinder, the blanket cylinder and the impression cylinder which presses the printing material against the blanket cylinder to transfer the printing inks and, for example for digital multicolor printing, one or more inking units. It is also known that the printing form or the forme cylinder can be provided in the printing machine with a thermal transfer film with thermal transfer material corresponding to an image. The transfer of the transfer material is preferably performed by laser induction, but other energy sources such as heating elements, heating lines, heating matrices or the like would also be conceivable. The surface of the forme cylinder is then inked with printing ink, in particular for an offset process and, as is known, the printing ink from the ink-carrying areas is transferred via the blanket cylinder onto the substrate or paper sheet to be printed. Known coverings for blanket cylinders for offset printing are, in particular, polyurethane materials, which are distinguished by a homogeneous surface, good dimensional stability and optimum ink and damping solution transport.
For a quick change of the printing motifs, in particular for small editions, the aim is to carry out the procedure within the printing machine, firstly under computer control and secondly without changing moving parts.
By means of an image-setting unit, the image information in the form of ink-accepting area elements on the lipophobic surface of the printing form or of the printing forme cylinder is transferred within the printing machine.
The thermal transfer film is provided with a thermally or electrothermally sensitive coating with oleophilic, that is to say ink-accepting properties. The image setting unit comprises a print head, which can be a row of heating elements, an electrode, an energy beam or any other unit that produces heat, in particular a laser print head. To transfer an item of image information, the print head is controlled via the appropriate image signals in such a way that it introduces heat and pressure into the thermal transfer film at each image point and therefore has the effect of transferring the coating of the film point by point to the surface of the forme cylinder. The forme cylinder rotates in the process, and the print head is traversed appropriately, so that the surface on the forme cylinder can have an image set on it from the thermal transfer film, for example in a spiral shape.
As is known, e.g. from U.S. Pat. No. 5,601,022, for the repeated coating of the forme cylinder surface in accordance with an image, subassemblies are arranged within the printing machine. These subassemblies include a feed device for a thermal transfer film to the forme cylinder, a laser print head that can be coordinated with the rotational movement of the forme cylinder, and an electronically controlled image-point transfer unit for activating the laser print head and an erasing or cleaning component which can remove the coating in accordance with the image from the surface of the forme cylinder again. As disclosed in U.S. Pat. No. 6,070,528, the element for removing the coating in accordance with an image or the thermal transfer material from the surface of the printing form may be a high-pressure cleaner, or else a blanket washing system, or cleaning by ultrasound or the like.
The strip-like thermal transfer film used in U.S. Pat. No. 5,601,022 can have a comparatively thin coating with thermal transfer material, so that the image-setting layer thickness on the forme cylinder can be thin and this coating in accordance with an image can also be removed again easily, that is to say the forme cylinder can be erased again or changed over more easily or more quickly, in accordance with the reduction in the layer thickness of the thermal transfer material.
Hitherto, in the case of printing units of the type previously described in use in small edition color printing (digital offset press), that is to say with printing forms which are coated in accordance with the thermal transfer process, use is made of forme cylinders with a metallic surface made, for example, of aluminum, of steel or steel alloys, of chromium or a metal oxide, since such surfaces are distinguished by good impression constancy. In order to print on the substrate or on the sheet in a small sheet machine, hitherto a blanket cylinder with a soft surface has been interposed and the substrate or the sheet has been led through between the blanket cylinder and an impression cylinder. Both the blanket cylinder and the impression cylinder are generally covered with a rubber blanket.
EP 0 368 180 B1 discloses a three-cylinder printing unit for an offset printing machine, in which the blanket cylinder is used as a transfer cylinder for a printing image produced by the surface of a forme cylinder by means of a thermal transfer process. The forme cylinder has an elastomeric surface with preferably hydrophilic property, the elastomeric surface of the forme cylinder being necessitated by the fact that fluctuations in linearity along the recording line can effectively be compensated for by its compressibility.
On the basis of the printing unit or machine configuration described above, it is the object of the invention to achieve further cost advantages and miniaturization effects for such a printing unit, in particular for use in small edition color printing (digital offset press).
The fact that the forme cylinder and blanket cylinder are combined into one cylinder in a printing unit of the generic type, in such a way that, in order to produce the reversible setting of an image (form production), this cylinder interacts directly with an impression cylinder in order to print on a substrate, and that image-setting unit and erasing and cleaning component and inking unit are set against this cylinder, so that this cylinder has the function of a forme cylinder, it is possible, for example, for the previous forme cylinder to be omitted (dilitho) and for the previous blanket cylinder to assume the function of the previous forme cylinder as well.
This means, in particular, as a particular cost advantage, the saving of a cylinder involved directly in the printing in a printing unit described at the beginning. Furthermore, the printing unit according to the invention provides a contribution to miniaturization with the objective of a xe2x80x9cPICOFORMATxe2x80x9d very small printing machine. In this way, the changeover costs are made cheaper and the required floor area is reduced.
A particularly preferred exemplary embodiment includes the fact that in order to produce the reversible setting of an image (form production), this cylinder is designed with a surface made of a hydrophilic elastomer, that is to say a polymeric material with a high elasticity, and the impression cylinder is designed with a comparatively hard surface made of rubber, but can also be made of steel.
A further advantageous embodiment is for the cylinder, which has both the function of a blanket cylinder and that of a forme cylinder, to be designed as a blanket cylinder with a conventional surface, that is to say with an incompressible (conventional) rubber blanket cover or even with a hard steel surface (erasable sleeve) and for the impression cylinder to be provided with a comparatively soft surface, in particular one made of a hard resilient rubber.
The fact that at least the hydrophilic surface of the cylinder accepting the printing image is designed without a seam, or preferably all the cylinder surfaces involved directly in the printing are designed without a seam means that very quiet running of the printing unit is ensured, since resonance phenomena arising from clamping channels or plate winding devices are ruled out.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.